Devices with bipolar and MOS transistors formed on a semiconductor substrate are disclosed, for example on pages 37-38 of "1989 Symposium on VLSI Technology".
These devices individually have at least one MOS transistor and at least one bipolar transistor in a P-type epitaxial layer which is formed on a semiconductor substrate. A region of the epitaxial layer, in which the bipolar transistor is formed, contains a well layer electrically isolating the MOS transistor and the bipolar transistor from each other.